The subject matter of this patent is related to the subject matter of earlier filed U.S. patent application Ser. No. 07/926,797, assigned to the assignee of the present invention, and relates generally to the art of compressing a gas in an oil-injected rotary screw compressor. More specifically, the present invention relates to apparatus for isolating rotor bearing lubricant passages and the oil injection port, which opens into the working chamber of an oil injected screw compressor, from their oil supply upon compressor shut down.
Screw compressors employed in refrigeration systems are comprised of complementary male and female screw rotors disposed within a working chamber defined by a rotor housing. The working chamber can be characterized as a volume generally shaped as a pair of parallel intersecting cylindrical bores and is closely toleranced to the outside length and diameter dimensions of the intermeshed screw rotor set. The rotor housing has low and high pressure ends which define unvalved suction and discharge ports in open-flow communication with the working chamber.
In operation, refrigerant gas at suction pressure enters the working chamber via the suction port and is enveloped in a chevron shaped pocket formed between the counter-rotating screw rotors. The pocket closes, its volume decreases and it is displaced toward the high pressure end of the compressor as the rotors meshingly rotate within the working chamber. The gas within such a pocket is compressed by virtue of the decreasing volume in which it is contained until the pocket opens to the discharge port at the high pressure end of the working chamber where it is expelled through the discharge port.
Due to the extremely close tolerances between the rotor set and the walls of the working chamber, the bearing arrangement in which the rotor set is mounted is critical to compressor operation and life. This is particularly true because the bearings and rotors in a screw compressor are subject to high and variable axial and radial loads. Protection and lubrication of rotor bearings is therefore of paramount concern in the design and operation of rotary screw compressors.
In addition to being delivered to the rotor bearings, oil is in many instances injected into the working chamber of a screw compressor through an injection port to perform several functions. First, the oil injected into the working chamber acts as a sealant between the rotors and the surfaces of the working chamber in which the rotors are disposed.
The oil also acts as a lubricant between the driving and driven screw rotor. In that regard, one of the two screw rotors is driven by an external source, such as an electric motor, while the other rotor is driven by virtue of its meshing relationship with the motor-driven rotor. Oil injected into the working chamber of the compressor therefore acts to prevent excessive wear between the driving and driven rotors.
Finally, injected oil is used to cool the refrigerant undergoing compression within the working chamber which in turn reduces the thermal expansion of the rotors that would otherwise occur as a result of the heat generated by the compression process. Such injection cooling therefor permits tighter rotor to housing clearances from the outset.
At compressor shut down, when the drive motor is de-energized, the backflow of discharge pressure gas from the high (downstream) side of the refrigeration system in which a screw compressor is employed back through the compressor discharge port, if allowed to occur, causes the high speed reverse direction rotation of the no longer driven screw rotors within the working chamber. Such reverse direction freewheeling of the rotors can occur at speeds greater than the maximum design RPM of the rotor set for normal operation.
Additionally, the resulting rush of downstream discharge pressure gas back through the compressor toward the low pressure side of the refrigeration system under such conditions is such that a relatively higher pressure can momentarily develop at the suction end of the compressor than exists at the discharge end of the compressor. This situation can result in the development of inordinate and uncommonly large axial forces on the screw rotor set and rotor bearings in a direction opposite that which is normally encountered and compensated for during compressor operation.
Also, many screw compressor bearing lubrication schemes are predicated on the development and maintenance of relatively high pressure downstream of the compressor which is used to drive lubricating oil from a sump or reservoir to the rotor bearings and/or injection port. The high speed reverse rotation of the rotor set at compressor shutdown and momentary development of relatively higher pressure at the upstream or low side end of the working chamber, if allowed to occur, can cause oil to be sucked from the bearings or not to be delivered to the bearings in sufficient quantity with potentially catastrophic results.
Finally, unless the oil injection port opening into the working chamber of a screw compressor is isolated from its typically pressurized oil supply upon compressor shutdown, oil will continue to flow through the injection port into the working chamber after shutdown, until the system pressures equalize, by virtue of the pressure differential which exists between the oil supply and the working chamber at compressor shutdown. Absent means for reliably isolating the oil injection port from its oil supply under such circumstances, the working chamber can become flooded with oil. As a result, excessive rotor deflection can occur and/or the compressor lubrication system can become starved for oil due to the dislocation of the oil supply from the oil sump to the working chamber. Under the first circumstance rotor to housing rubbing can occur while under the second circumstance insufficient oil may be available for delivery to the necessary locations within the compressor when the compressor next starts with potentially catastrophic results.
The need, therefore, continues to exist for a fail safe arrangement for preventing the continued flow of oil to the bearings and through the injection port into the working chamber of a refrigeration screw compressor upon compressor shut down and for permitting such oil flow at compressor startup.